In a conventional liquid-crystal display apparatus, in the case of a passive matrix liquid-crystal panel whose pixels do not have active elements, an image having intermediate gray shades such as a television image is displayed using a voltage averaging method.
However, using conventional liquid-crystal drive voltage waveforms entails the problem that crosstalk occurs on the display screen. The reason for this will be described below.
In the conventional matrix-addressed liquid-crystal display panel utilizing the voltage averaging method, N data electrodes S and M scanning electrodes T are arranged in a matrix form, each pixel being located at an intersection between the data electrodes S and the scanning electrodes T; the drive voltage waveform applied to each pixel corresponds to the difference between the drive voltage waveform applied to its associated scanning electrode T and the drive voltage waveform applied to its associated data electrode S.
When the matrix-addressed liquid-crystal display panel is driven to display a gray scale, noise is induced in the drive voltage waveform of the scanning electrode T at the rising and falling of the drive voltage waveform of the data electrode S because of the capacitive coupling between the scanning electrode T and the data electrode S. Since this noise is superimposed in such a manner as to reduce or increase the pulse in the drive voltage waveform applied to the pixel, the rms (root-mean-square value) voltage of the voltage waveform becomes smaller or larger than the ideal rms voltage.
The variation of the rms voltage value of the voltage waveform influences the occurrence of crosstalk. However, in the conventional matrix-addressed liquid-crystal panel, the rms voltage of the voltage waveform deviates in the increasing or decreasing direction with respect to the ideal rms voltage and, when these deviations are added up, excessive crosstalk occurs. In particular, in the case of liquid crystal operating in the STN mode, the influence that the variation of the rms voltage has on the occurrence of crosstalk is greater than in the case of TN mode liquid crystal.
To reduce such crosstalk, an improved driving method was proposed in Japanese Patent Unexamined Publication No. 62-183434. In the proposed driving method, fields where the rms voltage of the drive voltages applied to the liquid crystal during the non-selection period becomes smaller than the ideal rms voltage and fields where the rms voltage becomes larger than the ideal rms voltage are repeated in alternate fashion for every predetermined number of fields, thereby bringing the rms voltage of the voltage waveform as a whole closer to the ideal rms voltage.
However, in the liquid-crystal panel driven by such a drive voltage waveform, the rms voltage of the voltage waveform varies from one field to the next and a cycle of the large and small rms voltages is formed, the cycle thus becoming long and resulting in flicker on a screen.